CH 39: NUTRITION, DIGESTION, ABSORPTION
1. Animal Nutrition
2. Ways in which Animals obtain nutrients
3. Principles of Digestion and Absorption of Food
4. Overview of Vertebrate Digestive Systems
5. Mechanisms of Vertebrate digestion and absorption
6. Regulation of Digestion
7. Human Health Connection
- Nutrients are substances consumed by an animal that is needed for survival, growth, development, tissue repair, or
- Food is used in animals in 4 phases:
1. Ingestion: taking food into body via mouth
2. Digestion: nutrients in food is broken down into smaller molecules.
3. Absorption: Ions, water, small molecules diffuse or are transported to circulatory system
4. Elimination: Passing undigested material out of body.
39.1 Animal Nutrition
- There are 5 kinds of organic nutrients animals need (last semester)
- We also need inorganic nutrients – Ca2+ , K+, Fe3+ .
- Nutritional demands differ depending on animal’s physiology.
- Herbivores eat only plants so they have microbes to digest cellulose.
- Carnivores eat animal flesh or fluids
- Omnivores eat both plan and animals.
Animals Require Nutrients for Macromolecule Synthesis, Energy, and Chemical Reactions
- Minerals like Fe and Zn are needed as cofactors of enzymes and other proteins
- Ca 2+ are needed for bone, muscle, and nerve function
- Na and K establish ion gradients across plasma membranes and are important for heart, skeletal muscle, nerve activity.
- Amount of nutrients needed by an animal depends on its activity level/metabolic rate, many active animals – birds,
mammals – are endotherms, needing more nutrients.
- Exception: mammals that hibernate or normally sedentary animal temporarily expends more energy.
- Polymers consumed are usually too large to be absorbed, so they have to be broken into monomers blood and
cells synthesize new polymers.
Essential Nutrients Must be obtained from Diet
1. Essential Amino Acids
- 8 amino acids are required for protein synthesis in diet because they can’t be synthesized or stored.
- Unlike animal meat, plants do not contain many of these amino acids.
- Some herbivores, such as cows, have evolved the capacity to synthesize the essential amino acids.
2. Essential Fatty Acids
- Polyunsaturated fatty acid – linoleic acid – cannot be synthesized by animal cells, compared to saturated fatty acid,
which can be synthesized.
- Linoleic acid arachidonic acid precursor to production of several compounds important in aspects of physiology.
- Felines cannot synthesize arachidonic acid, obtain from fish and adipose tissue.
3. Minerals - Inorganic ions required to build skeletons, maintain balance of salts and water, provide source of electric current across
plasma membranes, exocytosis, and muscle contraction.
- Only trace amounts are needed because they can be stored.
o Ex. Iodine is stored in thyroid gland
o Ca 2+ stored in bone and can be released into blood
- Not all minerals are used the same way or at same rate
o Ex. Cu binds and transport oxygen in invertebrates but in all vertebrates and most other invertebrates, Fe
serves this function.
- Organic nutrients that serve as coenzymes for many metabolic and biosynthetic reactions
- 2 Categories:
1. Fat soluble: (Ex. Vitamin A) stored in adipose tissue
2. Water soluble: (Ex. Vitamin C) must be regularly ingested.
- Inadequate vitamin intake can cause problems – scurvy is a vitamin C deficiency.
39.2 Ways in which Animals Obtain Nutrients
Animals May Eat Plants, Other Animals or Both
- Autotroph: primary producers that harvest light or chemical energy and store it in carbon compounds
- Heterotroph: found at higher trophic levels, obtain nutrients from environment.
o Include herbivores, carnivores, omnivores.
o Not all heterotrophs are animals (ex. Venus Fly Trap and fungi – obtain nutrient from living (parasitic&
predatory) and dead matter) and not all plants are autotrophs.
- The gut of carnivores and omnivores resemble each other more than herbivores because of the enzymatic processing
and energetic quality of the foods they eat.
- Opportunistic animals have a preference for one type of food, but can adjust their diet if the need arises.
- Animal life stage also influence diet – caterpillars eat leaves but butterflies are fluid drinkers.
Animals have Evolved Multiple Strategies for Obtaining Food
- Sedentary aquatic animals may use sticky surfaces or tentacles to trap food.
- Suspension feeders sift water to filter out organic matter and Baleen whales have a keratin baleen that filters out small
animals and protozoa for ingestion.
- Carnivores are predators or scavengers - eat remains of dead animals – and use chewing as means of breaking food
into swallowable pieces.
- Herbivores have powerful jaw muscle and large teeth to grind fibrous plants. Grazers – grass eating – and frugivores
(fruit eating) use chewing to facilitate digestion of plant material.
- Fluid feeders lick or suck fluid from plants or animals, they don’t have teeth unless to puncture.
o Mosquitos, lamprey, leeches (these have anaesthetic in saliva that prevents clotting, and digests host’s
connective tissue to allow embedding of teeth)
39.3 principles of Digestion and Absorption of Food
Digestion can occur Intracellularly or Extracellularly
- Intracellular digestion occurs only in simple invertebrates – sponges and single-celled organisms (protozoa)
- Uses phagocytosis where food is segregated from cytoplasm in food vacuoles so it can be digested by hydrolytic
- How do membranes fuse? Perhaps negative charges on phospholipids have to be neutralized using Ca2+.
- Intracellular digestion can only phagocytose tiny bits of food and cannot store large quantities of food.
- Extracellular digestion protects interior of cells from hydrolytic enzymes and allows animals to consume large prey or
large pieces of plants.
o Simplest form: gastrovascular cavity (digestive and circulatory movements occur) has 1 opening as entry and
exit. o Ex. Hydra phagocytose digested food into cells that line gastrovascular cavity.
Most Digestive Cavities are tubes with Specialized Regions and Openings at Opposite ends
- More complex organisms have alimentary canal with openings at both ends, it is lined with epithelial cells to make
digestive enzymes, secrete hormones to regulate digestion.
- The foregut (stomodeum) is place of ingestion and hindgut (proctodeum)is the place of secretion are relatively
- Midgut is major site of enzyme production and secretion, digestion and absorption, it contains structures that increase
internal surface area – pouches and outcroppings.
Absorption of Food can Be Passive or Active
- Nutrients are absorbed by alimentary canal cells by passive/facilitated diffusion or active transport.
- Hydrophobic molecules (fats) passive diffusion across epithelium
- Ions and other molecules facilitated diffusion/active transport.
- Hydrophilic molecules secondary active transport with Na+
- After nutrients enter epithelial cells of alimentary canal, the cells use some of these nutrients for own needs but most
nutrients go to blood vessels where they circulate to other cells.
39.4 Overview of Vertebrate Digestive Systems/ 39.5 Mechanisms of Digestion/Absorption in vertebrates
Alimentary Canals are divided into functional regions
- The alimentary canal (aka gastrointestinal tract) has 3 parts:
1. Anterior end to ingest food, including the oral cavity, salivary glands, pharynx, and esophagus.
2. Middle portion stores and initial digestion of food, includes the crop, gizzard, and stomach, depending on species,
and the upper part of small intestine with accessory organs : pancreas, liver, gallbladder
3. Posterior part does final digestion absorption, and elimination of wastes. Consists of small intestine, rectum (stores
feces) and anus (eliminates feces)
- The hollow cavity from the midesophagus anus is the lumen and it’s lined by epithelial cells. These cells release
mucus coating into lumen, hormones into blood, and the glands secrete acid, enzymes, water, and ions.
- Epithelial cell layer is surrounded by smooth muscle to churn stomach/intestine contents, nerves to detect food
sight/smell, and connective tissue.
- Herbivores chew extensively while carnivores don’t chew at all unless to slice flesh.
Where it’s Food Processors What’s secreted/ Function
found used enzymes used
Mouth Saliva Amylase - Moistens and lubricates food to facilitate swallowing
- Dissolve food particles so we can taste it.
- Kills bacteria with antibodies
- Initiate digestion using amylase.(pH 7)
o Starch maltose
o Anticoagulant properties (Tsetse fly)